The modern semiconductor industry relies on potential to change properties of materials by introduction of impurities and creation or elimination of structural defects in lattices. The predominant type of defect is a vacancy created when chemical bonds between neighboring atoms are broken. A vacancy is a relatively long-lived object, characterized by absence of an atom from a lattice node. Vacancies can be created by ionizing radiation, either directly (when a high-energy massive particle “knocks out” an atom from a lattice node into an interstitial position), or indirectly, when the atom is at first excited due to Auger effect caused by X-rays, whereupon the double ionization of the atom causes a change of the force of its interaction with neighbouring atoms and an increase of the probability that the atom will leave the lattice node due to random thermal motion (this is how X-ray radiation creates vacancies). Diffusion of vacancies is caused by thermally-stimulated “jumps” of atoms into neighbouring vacant positions (i.e., vacant and occupied positions of lattice nodes are interchanged, which is equivalent to spatial motion of the vacancy).
The broken chemical bonds responsible for existence of a vacancy are called “dangling bonds”. Vacancies greatly facilitate diffusion of dopant atoms. Normally, a sufficiently high rate of diffusion can only be achieved when the material is heated to a high temperature (approximately 1000°). Vacancies can lower the required temperature substantially. However, in order to be able to apply this property in practice, vacancies must be sufficiently long-lived (i.e., metastable). At favorable conditions, extremely fast diffusion (the so-called “superdiffusion”) can be achieved even at room temperature.
During investigation of nonlinear diffusion in excited systems (A. J. Janavi{hacek over (c)}ius, Lith. J. of Phys., 37, 508-510 (1997)), attention was drawn to fast thermal diffusion of indium in a p-type HgCdTe/CdTe crystal excited by ultraviolet-radiation photons with energy of 4.14 eV emitted by a mercury-halogen lamp. This very fast diffusion was discovered experimentally and described in the paper Seung-Man-Park et al, Jpn. J. Appl. Phys., 35, 1554 (1996). By applying the method of secondary ion mass spectroscopy, indium fast diffusion profiles corresponding to extremely low activation energy have been measured (the measurements were carried out at temperatures of 120° C., 150° C., 180° C.). Since only about 1% of indium atoms participated in fast diffusion, no practical application for the discovered phenomenon was found, despite the fact that those atoms reached depth of 12 μm faster than in 30 s. However, those results have a theoretical value, because they demonstrate the possibility to initiate superdiffusion using photon radiation.
One of current topics of research is superdiffusion initiated by particle beams, when impurities are being introduced after bombarding the sample with fast electrons, protons and slow neutrons. This type of superdiffusion can be only partially considered to be diffusion, because it involves knocking out of atoms from nodes of a crystal lattice. Neutron beams have been used to introduce impurities during manufacture of diodes and solar cells, but p-n junctions obtained by this method have many defects and are close to a surface of a sample. Fast particles damage a lattice, create a multitude of defects and change the surface structure, they may cause formation of an amorphous layer at a surface of a crystal. Consequently, applications of superdiffusion initiated by particle beams for manufacture of electronic devices are currently of purely experimental character.
We used soft X-ray radiation for creation of metastable vacancies in Si lattice by means of the Auger effect. We discovered experimentally boron and phosphorus superdiffusion via metastable vacancies in crystalline silicon at a room temperature. On the basis of superdiffusion of this type, we can propose promising technologies for manufacturing of high-quality electronic devices (photodiodes, solar cells).
This important conclusion was made by comparing our results with superdiffusion initiated by fast electrons. Control of doping profiles using fast electron technology is described in the paper by Takao Wada and Hiroshi Fujimoto “Electron Beam Doping of Impurity Atoms into Semiconductors by Superdiffusion, Phys stat. sol. (c) 0, No. 2, 788-794 (2003)”. The investigations described in that article were performed with extremely thin diffusion layers (layer thickness 50 to 400 Å), which are difficult to apply in practice. Fast-diffusing and long-lived (at room temperature) vacancies generated with soft X rays (A. J. Janavi{hacek over (c)}ius, J. Banys, R. Purlys, and S. Balakauskas, Lithuanian Journal of Physics 42, No 5, 337 (2002)) can be used for deep doping of semiconductors. The nonlinear diffusion theory, presented in the article A. J. Janavi{hacek over (c)}ius, Phys. Lett. A., 224, 159-162, (1997), suggests new experimental methods of investigation of point defects, using electric and Bragg diffraction measurements. The method of superdiffusion in excited systems, which was presented in the article A. J. Janavi{hacek over (c)}ius, Acta Phys. Pol. A 93, 505 (1998), is fundamentally different from superdiffusion initiated by fast electrons and other particles. Superdiffusion, which is initiated by fast electrons (1-2 MeV), protons, gamma rays or other particles, not only creates vacancies and Frenkel pairs, but it also introduces oxygen complexes, which damage the region of the sample and render it unsuitable for manufacture of electronic devices. When crystals are exposed to fast particles, a large amount energy is transferred to the lattice. This energy contributes to the energy of lattice vibrations, decreases the time of existence of vacancies and the probability to participate in diffusion. Vacancies that are generated by soft X-rays are long-lived (more than 1.5 h) at a room temperature and can be applied in practice. Generation of superdiffusion of impurities in semiconductors using soft X-rays with the aim to create novel devices (photodiodes, solar cells) is a complex process, which depends on properties of a crystal, type of impurities used and vacancy charges. Therefore, further theoretical and experimental studies are needed. Long-lived and fast-diffusing vacancies are the reason of phosphorus and boron superdiffusion in p-type silicon crystals at room temperature. The resonances of vacancy dangling bonds under action of alternating radio-frequency electric field are fundamentally new phenomena, which are presented for patenting herein.
A known doping method, which is described in the U.S. Pat. No. 4,824,798, is based on generation of vacancies at a high temperature (up to 900° C.) using an oven. The drawback of this method is a large expenditure of energy and time (approximately 8 hours).
A method of manufacturing light-emitting diodes, which is described in the U.S. Pat. No. No. 4,639,275, is based on diffusion of zinc ions at a high temperature (750° C. to 1050° C.) in a heterojunction formed of III-IV semiconductors (in this case, those ions are needed for creation of a disordered layer and a consequent increase of the energy gap of the material).
The U.S. Pat. No. 4,843,033 describes a different method of zinc ion diffusion into GaAs substrates, which is based on deposition of a zinc tungsten silicide (WxSi:Zn) layer on GaAs surface and heating at a temperature of 600° C. to 700° C.
Another widespread method of semiconductor doping is ion implantation. Its main drawback is the fact that high-energy ions knock the atoms out of a lattice nodes, i.e., damage the crystal lattice. In order to restore regular positions of atoms, annealing is necessary after the implantation (a typical annealing temperature is approximately 800° C.). Some implantation techniques are designed to minimize the damage done to the semiconductor due to implantation. For example, the U.S. Pat. No. 4,927,773 describes a method based on coating the semiconductor with a layer of another material, which is used as a target of ion implantation, whereupon thermal diffusion from that additional layer into the semiconductor is carried out. In this case, the additional layer protects the semiconductor from damage, and the annealing temperature can be as low as 250° C. However, the annealing time is still rather long—up to 16 hours.
The closest technical solution, which has been chosen as the analogue of this invention, is the method of vacancy generation during doping processes using X-ray radiation. Using this method, the following operations are performed: a layer consisting of the chemical element that is to be introduced into the bulk of a material is deposited on a crystalline sample of the just-mentioned material, then the crystal is exposed to X-rays (A. J. Janavi{hacek over (c)}ius, S. Balakauskas, V. Kazlauskienė, A. Mekys, R. Purlys and J. Storasta. Superdiffusion in Si Crystal Lattice Irradiated by Soft X-Rays. Acta Phys. Polon. A. ISSN 0587-4246. Vol. 114, No. 4 (2008), p. 779-790). Vacancies generated by soft X-rays are characterized by fast diffusion and long lifetimes at a room temperature (A. J. Janavi{hacek over (c)}ius, J. Banys, R. Purlys, and S. Balakauskas, Lithuanian Journal of Physics 42, No 5, 337 (2002)), hence they can be used for deep doping of semiconductors.
The drawback of this method is that vacancies diffusion only takes place at specific operation conditions of the X-ray tube (anode voltage of 9 kV and anode current of 23 mA). The method is very sensitive to X-ray tube anode current and voltage as well as to X-ray flux incident on the surface. As anode current grows to 30 mA, diffusion of vacancies decrease several times and eventually disappear completely at larger values of anode current. Diffusion of vacancies also disappear after increasing voltage by 1-2 kV. As a result, gradual change of equipment parameters with time, which causes a corresponding change of radiation flux, makes it difficult to find the optimal operation conditions. Besides, when vacancies are created by this method, the process of their diffusion into the bulk of the exposed material takes from 1 to 6 hours. Our invention includes using a high-frequency electric field, which acts upon the material exposed to X-ray radiation. It is used for fast saturation of vacancy concentration in a material in order to create favorable conditions for diffusion of impurities. This method makes it possible to achieve fast diffusion of impurities in any material, because there is no need for heating it. Since diffusion of impurities is fast, the destructive effect of radiation is insignificant. In comparison with the closest analogue, the proposed new method of speeding up the diffusion of impurities is more technological and has a wider area of potential applications. Using superdiffusion of this type, it is possible to propose promising technologies for manufacture of high-quality electronic devices (photodiodes, solar cells).